PPC: improve handling of improper "kind-of-register" operands

A unary % is expected to be followed by a register. While this is being
diagnosed as "bad expression", a 2nd error then follows ("syntax error;
found ..."). Consume the % in md_operand() (thus preventing it to be
treated as modulus with missing [i.e. implicitly 0] leading operand) and
mark the expression O_illegal (thus avoiding the "bad expression") when
the "register" is unrecognized.

Reviewed-by: Peter Bergner <bergner@tenstorrent.com>